1. Field of the Invention
This invention relates generally to scanning radio receivers and in particular to an improved channel scanning radio receiver which automatically monitors an activated channel with priority without stopping the scan function.
2. Description of the Prior Art
Scanning radio receivers which automatically and sequentially scan a plurality of channels for tuning to predetermined radio frequencies, lock on to a tuned channel receiving a signal, and upon termination of the received signal automatically resume the scan to locate another channel receiving a signal are now well known. In addition, some systems utilize a priority scanning operation in which a priority is assigned to one of the channels and in some systems a secondary priority is assigned to yet another channel. In a system having such a priority channel it is necessary to continuously sample the priority channels during the reception of signals on other channels and to lock onto the priority channel whenever a carrier is detected during a sampling interval. Such a system is disclosed in U.S. Pat. No. 3,614,621 issued on Oct. 19, 1971 to Ron Chapman, et al and assigned to Motorola, Inc.
Such channel scanning radios have several major limitations which make them cumbersome and unsuitable for two way communications. One attempt to make two way communications possible on a channel, for example, to answer an incoming call detected by the channel scanner, requires freezing the scanner on the channel when the microphone is removed from its holder. However, the microphone must be manually left off hook for a period of time to ensure any further communications will be received. Thus, either the scanning must be stopped with possible loss of desired information on another channel, or the user risks loss of any unanticipated further communications on the channel. Another approach uses a delay in resumption of scanner function following termination of a signal on a receiving channel. This however results in dramtically slowing down the scanning with resultant loss of communications. Consequently, the prior art does not permit the user to conveniently follow the progress of communications on a desired channel during periods of frequent channel activity without losing the advantages of the scanning function.